1. Field of the Invention
The present invention relates to a zoom lens having a plurality of wobbling lens units and an image pickup apparatus equipped with such a zoom lens.
2. Description of the Related Art
A zoom lens having a wobbling lens unit has already been known. The wobbling lens unit is a lens unit which minutely oscillates (wobbles) at high speed along the direction of the optical axis to change the contrast of the image to an extent that is not recognizable in the reproduced image, and the focus state on the image pickup element is detected utilizing the oscillation (wobbling) of the wobbling unit.
For example, when an object is in focus, while the contrast of a formed image changes slightly with oscillation (wobbling) of the wobbling lens unit, the image shows substantially the same contrast at both ends of the wobbling range.
On the other hand, when the object shifts forward or backward to be out of focus, the image has different contrasts at both ends of the wobbling range. The object's going out of focus is detected from this change in the contrast, and the focus lens unit is shifted along the optical axis to bring the object into focus again. Thereafter, the wobbling lens unit is caused to oscillate minutely at high speed again to maintain the in-focus state.
The period of oscillation of the wobbling lens unit depends on the frame rate of the system including the image pickup element. Japanese Patent Application Laid-open Publication No. 2007-293146 describes a technique of changing the amplitude of oscillation of the wobbling lens unit in accordance with the zoom state to cause an appropriate change in contrast irrespective of the zoom state.